1. Field of the Invention
The invention relates to surface plasmon resonance sensors based on plasmonic grating and sensitivity enhancement can be obtained after an azimuthal rotation of gratings. The method describes a device and procedure to exploit this higher sensitivity.
2. Description of the Related Art
Sensors are devices for detecting and measuring physical, chemical and biological quantities. Sensors can be grouped into electrical, optical and mechanical sensors in accordance with various detection mechanisms. First optical chemical sensors were based on the measurement of changes in absorption spectrum and were developed for the measurement of CO2 and O2 concentration. Since then a large variety of optical methods have been used in chemical sensors and biosensors including, spectroscopy (luminescence, phosphorescence, fluorescence, Raman), interferometry (white light interferometry, modal interferometry in optical waveguide structures), spectroscopy of guided modes in optical waveguide structures (grating coupler, resonant mirror), and surface plasmon resonance (SPR). This discovery deals with the last type of sensors and, in particular, describes a device and method for the enhancement of the index refraction sensitivity based on the control of azimuthal angle of rotation of 1D plasmonic gratings
Surface plasmon polariton (SPP) is defined as an electromagnetic (photon) excitation that couples with the electrons oscillations (on thin metal film) and propagates as a wave (polariton) along the interface between a metal and a dielectric medium. Fields intensity decays exponentially from the surface with extension length of the same order of wavelength inside the dielectric medium and about one order shorter into the metal. Due to this phenomenon, SPPs are particularly sensitive to optical and geometrical properties of the surface, e.g. shape, profile, roughness, refractive indices, and reveal themselves as a useful tool for surface analysis. These light-matter interactions and sensitivity due to the field enhancement are extensively used for chemo- or bio- sensing purposes. The resonant condition for excitation of surface plasmons with an electromagnetic wave depends on refractive index of the dielectric in the proximity of the metal surface. Therefore, variations in the refractive index can be monitored from changes in the interaction between an electromagnetic wave and a surface plasmon.
SPP sensors typically measure shifts of surface plasmon resonance as a function of a change of a refractive index of analyte molecules or a chemo-optical transducing medium. In optical sensors, surface plasmons are usually optically excited with an electromagnetic wave in the visible or near infrared spectrum.
SPR sensors can be used also as highly sensitive refractometers and can also be applied for the study of biomolecules and their interactions and for detection of chemical and biological compounds. In these applications, SPR sensors are combined with bio/chemo recognition elements which specifically interact with an analyte (e.g., antibody, enzymes, DNA).
Currently, several groups are using different SPR approaches to detect the change of refractive index. A refractive index resolution better than 3×10−7 RIU (refractive index units) has been developed by Liedberg and BlAcore using a Kretschmann configuration prism-coupled SPR (PC-SPR) sensor; this study also concluded that sensitivity is higher at short wavelength. Gaurav claimed an angular sensitivity from 94.46°/RIU to 204.41°/RIU based on changing the prism refractive index. Van Duyne and his coworkers, working on localized surface plasmon resonance (LSPR) of noble metal nano-particle arrays, reported a refractive index resolution of 5×10−3RIU. Perez-Juste and Yu used gold nanorods with an aspect ratio of 3 to build multiplex biosensors with a sensitivity of 400 nm/RIU. Although prism-based coupling methods provide the best refractive index resolution, they suffer from extremely cumbersome optical maintenance. Furthermore, the commercialized prism-based SPR instrument is very expensive and is not amenable to miniaturization and integration. A LSPR nanosensor is much cheaper than an SPR instrument and can be miniaturized, but it has much lower sensitivity. Sensing based on Au nanorod SPR is a new method; a systematic analysis of the sensitivity has not yet been presented and controlling the aspect ratio of nanorod is inconsistent and troublesome.
Another common way for SPR excitation is to use a metallic grating. Yoon and Cullen have proposed grating coupled SPR (GC-SPR), demonstrating a sensitivity of 440 nm/RIU or 100°/RIU, which is lower than prism-based coupler SPR. Most of the groups are using a two dimensional CCD array to collect the reflected light from the grating substrate, which provide a higher reflective index resolution of ˜10−6 RIU and over 200 sensing channels. Recently, Homola's group demonstrated an SPR biosensor with a reflective index resolution of 3.5×10−6 RIU by using the advantages of both long-range and short-range surface plasmon excited simultaneously on a diffraction grating. Most recently, Alleyne has demonstrated a higher sensitivity of 680°/RIU by bandgap-assisted GC-SPR, but this requires a prism to enhance incident light momentum for exploring the grating's bandgap region.
Recently, our group has shown that the number of excited SPP modes in GCSPR is related to the azimuthal angle of the grating.
A SPP spectroscopy method of in surface plasmon resonance sensors was described in patent US 2008/0144027 A1 and in patent US 2007/0279634 A1 exploiting SPP grating based sensors and also the rotation of the gratings but with a different detection approach. When SPP is excited at a particular incident wavelength or angle, a dip in reflectivity spectrum can be observed.